The present invention relates to a gas concentration sensing apparatus using a gas concentration sensor capable of detecting the concentration of a specific component contained in an exhaust gas emitted from an automotive engine.
Exhaust gases emitted from automotive engines cause the air pollution which induces serious problems in the modern society. To reduce the harmful or poisonous substances contained in the emission gases, the law regulations for emission gas purification standard have been becoming severe year by year. Thus, the study and development for suppressing the emission amount of harmful or poisonous substances contained in the exhaust gas has been steadily conducted through the improvement of combustion control for gasoline or diesel engines and the use of catalytic converters. The regulation OBD-II (on Board Diagnostic-II) is already introduced in the United States, according to which every purification system is required to check whether a catalytic converter is in an appropriate condition for purifying the emission gases.
There is a so-called 2 O2 sensor monitoring system including one O2 sensor disposed at an upstream side of a catalyst and another O2 sensor disposed at a downstream side. This system is, however, an indirect detecting system. No pollution substances are directly detectable by this system. In other words, it is impossible to accurately check wether the pollution substances in the exhaust gas are actually reduced.
If a combustion control monitoring or a catalyst monitoring becomes feasible by directly monitoring a NOx concentration in the exhaust gas, this will make it possible to accurately check the reduction of the pollution substances in the exhaust gas. Namely, if the fuel injection or the EGR rate is feedback controllable by a NOx concentration value detected from the exhaust gas, it will become possible to reduce the pollution substances emitted from engines. Furthermore, providing a NOx sensor at a downstream side of the catalytic converter is effective to easily check a deteriorated condition of a catalyst accommodated in the catalytic converter.
From the foregoing, it is desirable to provide a NOx sensor capable of accurately detecting the NOx concentration in the exhaust gas. And also, it is desirable to provide a technique for installing the NOx sensor in an automotive engine.
This kind of conventional technique is, for example, disclosed in the unexamined Japanese patent publication No. 8-271476 (corresponding to the U.S. Pat. No. 5,866,799) or in the unexamined Japanese patent publication No. 9-318596 (corresponding to the European Patent Application No. 798,555), according to which the oxygen concentration is reduced in advance to detect the NOx concentration. More specifically, a chamber is provided separately from the exhaust gas environment via a diffusive resistor. A pump cell discharges the oxygen (O2) from this chamber. A sensor cell decomposes and discharges the residual NOx. A current value obtainable from the sensor cell indirectly represents the NOx decomposition amount, i.e., the NOx concentration. To activate the NOx sensor, it is necessary to keep the temperatures of the pump cell and the sensor cell at a predetermined level or above so that the oxygen ion conductivity of a solid electrolytic element can be enhanced. To this end, the NOx sensor comprises a heater provided in the vicinity of the pump cell and the sensor cell for warming up these cells.
Furthermore, when detecting the exhaust gas components, the NOx sensor is subjected to a large temperature fluctuation as well as a large gas flow fluctuation of the exhaust gas. Accordingly, to improve the accuracy of the NOx sensor, it is necessary to accurately control the temperature of the NOx sensor to a constant value.
For example, a first method for controlling a sensor temperature is to detect a heater resistance and control the sensor temperature in accordance with a detected heater resistance value. According to this method, a heater temperature is measured indirectly by measuring the heater resistance value. The measured temperature is regarded as being substantially equal to the cell temperature. And, the temperature control is performed so as to maintain the heater temperature at a constant level. A second method is to detect an internal impedance of a pump cell and control the sensor temperature in accordance with a detected internal impedance value. Usually, an impedance of the pump cell is detected to control the heater. For example, SAE-No. 970858 discloses a heater control of a sensor comprising first and second pump cells. According to this conventional heater control, an impedance of the second pump cell is detected and the heater is controlled based on a detected impedance value.
However, the above-described first method is inaccurate in controlling the sensor cell temperature when the exhaust gas temperature varies widely or the exhaust gas flow speed is high, because the cell temperature is different from the heater temperature in such conditions. Similarly, the above-described second method is inaccurate in controlling the sensor cell temperature when the exhaust gas temperature varies widely or the exhaust gas flow speed is high, because the pump cell temperature is different from the sensor cell temperature in such conditions.
Accordingly, the sensor cell temperature fluctuates and the NOx concentration sensing accuracy worsens. It is thus desirable to provide a sensing apparatus capable of accurately detecting a gas concentration regardless of the temperature fluctuation or gas flow speed change in the exhaust gas.
In view of the foregoing problems in the prior art, the present invention has an object to provide a gas concentration sensing apparatus which always assures an accurate gas concentration detection regardless of the temperature fluctuation or gas flow speed change in the measuring gas.
In order to accomplish this and other related objects, the present invention provides a gas concentration sensing apparatus using a gas concentration sensor comprising a first cell for discharging excessive oxygen contained in a measuring gas in accordance with an applied voltage and producing a current responsive to an oxygen concentration, a second cell producing a current responsive to a concentration of a specific component involved in the residual measuring gas after the excessive oxygen is discharged, and a heater for heating the first cell and the second cell. For example, when this gas concentration sensing apparatus is used to detect both the oxygen concentration and the NOx concentration in an exhaust gas, the first cell detects the oxygen concentration and the second cell detects the NOx concentration.
According to a first gas concentration sensing apparatus in accordance with the present invention, an internal resistance of the second cell is detected. And, electric power supplied to the heater is controlled in accordance with a detected internal resistance value of the second cell.
The above-described arrangement makes it possible to control the internal resistance of the second cell to a desired value constantly through the electric power control of the heater. Accordingly, it becomes possible to prevent the temperature of the second cell from being changed undesirably due to the temperature or flow speed fluctuation of the measuring gas (e.g., exhaust gas), thereby appropriately maintaining the NOx concentration sensing accuracy. Furthermore, as apparent from the characteristics shown in FIG. 6, the NOx concentration is detectable in a relatively narrow region (i.e., a flat region). The temperature variation of the second cell (e.g., a sensor cell) renders the sensor output unstable. However, the above-described heater power control ensures an accurate detection of the NOx concentration. As a result, the present invention makes it possible to always assure an accurate gas concentration detection regardless of the temperature fluctuation or the gas flow speed change of the measuring gas.
Preferably, an internal resistance of the first cell is also detected. And, the voltage applied to the first cell is controlled in accordance with a detected internal resistance value of the first cell.
As described above, when the heater power control is performed based on the internal resistance of the second cell (e.g., sensor cell), the temperature of the second cell (e.g., sensor cell) can be controlled to a constant value. However, the temperature of the first cell (e.g., pump cell) may vary in accordance with the temperature change of the measuring gas. To solve this problem, it is preferable to control the voltage applied to the first cell in accordance with the internal resistance of the first cell. This arrangement makes it possible to adequately manage the applied voltage. The oxygen concentration sensing accuracy can be maintained appropriately. The sensing accuracy of the NOx concentration is also improved in accordance with the improvement of the oxygen concentration sensing accuracy.
The present invention further provides a second gas concentration sensing apparatus using a gas concentration sensor comprising a plurality of cells including a first cell for discharging excessive oxygen contained in a measuring gas in accordance with an applied voltage and producing a current responsive to an oxygen concentration, and a second cell producing a current responsive to a concentration of a specific component involved in the residual measuring gas after the excessive oxygen is discharged, and a heater for heating the plurality of cells. The second gas concentration sensing apparatus is characterized by detecting means for detecting an internal resistance of each of the plurality of cells, judging means for judging temperature conditions of the plurality of cells, and power control means for selectively performing a heater power control based on a detected internal resistance value with reference to the judgement result of the temperature conditions.
Selectively performing the heater power control based on the internal resistance of the plurality of cells makes it possible to suppress the deterioration of the sensing accuracy in the gas concentration detection. Thus, the gas concentration sensing accuracy can be maintained adequately even when respective cells have temperatures different from each other due to their structures.
Practically, it is desirable that, in a cold startup condition, the heater power control is performed based on a detected internal resistance value of a highest temperature cell among the plurality of cells. And thereafter, the heater power control is performed based on a detected internal resistance value of the second cell. Alternatively, it is desirable that the heater power control is performed based on a detected internal resistance value of a highest temperature cell among the plurality of cells when there is a large temperature difference among the plurality of cells. And, the heater power control is performed based on a detected internal resistance value of the second cell when there is a small temperature difference among the plurality of cells.
According to this arrangement, when the temperature of the measuring gas (e.g., exhaust gas) is stable, the heater power control is performed based on the internal resistance of the second cell. When the temperature of the measuring gas is temporarily increased, the heater power control is performed based on the internal resistance of the cell having the highest cell. Hence, it becomes possible to adequately maintain the gas concentration sensing accuracy even when the ce; temperature distribution varies.
A third gas concentration sensing apparatus in accordance with the present invention comprises first detecting means for detecting an internal resistance of the first cell, second detecting means for detecting an internal resistance of the second cell, and power control means for controlling electric power supplied to the heater so as to equalize a sum or an average of detected internal resistance values of the first and second cells with a target value.
In this case, even when the temperature distribution varies in each cell, the temperature difference can be reduced effectively. As a result, it becomes possible to maintain each cell in an appropriate temperature zone so as not to be excessively heated or cooled, thereby enabling a stable gas concentration detection.
The second or third gas concentration sensing apparatus may further comprise voltage control means for controlling the voltage applied to the first cell based on the detected internal resistance value of the first cell. In this case, it is preferable to control the voltage applied to the first cell in accordance with the detected internal resistance value of the first cell. The applied voltage can be adequately managed. The oxygen concentration sensing accuracy can be maintained appropriately.
In the first to third gas concentration sensing apparatus, the internal resistance of each cell is detected by temporarily changing the voltage or current applied to each cell. A sample hold circuit is provided in a signal path for outputting a sensor signal representing a detected oxygen or other gas concentration in the measuring gas. The sample hold circuit holds a latest value of the sensor signal during the internal resistance detection of each cell.
Namely, the gas concentration sensing apparatus temporarily changes the voltage applied to respective cells including the first and second cells to detect their internal resistance values. Such a voltage change may cause an interference of currents flowing through respective cells. The output signal representing a detected gas concentration may fluctuate undesirably. To solve this problem, the present invention provides the sample hold circuit for holding the latest value of the sensor signal during the internal resistance detection of each cell.
The first to third gas concentration sensing apparatus may further comprise speed limiting means for limiting a change speed of the voltage applied to each cell. This is effective to suppress the oscillation of the applied voltage.